1. Slow down
2. Check your tire pressureUnder-inflated tires are one of the most commonly ignored causes of crummy MPG. Tires lose air due to time (about 1 psi per month) and temperature (1 psi for every 10 degree drop). Under-inflated tires have more rolling resistance, which means your engine has to work harder to keep your car moving. Buy a reliable tire gauge and check your tires at least once a month. Be sure to check them when they are cold, since driving the car warms up the tires (and the air inside them), which increases pressure and gives a falsely high reading. Use the inflation pressures shown in the owner's manual or on the data plate in the driver's door jamb.
3. Check your air filterA dirty air filter restricts the flow of air into the engine, which harms performance and economy. Air filters are easy to check and change: See your owner's manual for instructions. Remove the filter and hold it up to the sun; if you can't see light coming through it, you need a new one. Consider a K&N or similar "permanent" filter which is cleaned rather than changed. They provide better airflow than throw-away paper filters and they're better for the environment.
4. Accelerate with careJack-rabbit starts are an obvious fuel-waster -- but that doesn't mean you should crawl away from every light. If you drive an automatic, accelerate moderately so the transmission can shift up into the higher gears. Stick-shifters should shift early to keep the revs down, but don't lug the engine; downshift if you need to accelerate. Keep an eye well down the road for potential slowdowns. If you accelerate to speed then have to brake right away, that's wasted fuel.
5. Hang with the trucksEver notice how, in bad traffic jams, cars seem to constantly speed up and slow down, while trucks tend to roll along at the same leisurely pace? A constant speed keeps shifting to a minimum -- important to big-rig drivers who have to wrangle with those ten-speed truck transmissions -- but it also aids economy, as it takes much more fuel to get a vehicle moving than it does to keep it moving. Rolling with the big rigs saves fuel (and aggravation).
6. Get back to natureConsider shutting off the air conditioner, opening the windows and enjoying the breeze. It may be a tad warmer, but at lower speeds you'll save fuel. That said, at highway speeds the A/C may be more efficient than the wind resistance from open windows and sunroof. If I'm going someplace where arriving sweaty and smelly could be a problem, I bring an extra shirt and leave early so I'll have time for a quick change.
7. Back off the blingNew wheels and tires may look cool, and they can certainly improve handling. But if they are wider than the stock tires, they'll create more rolling resistance and decrease fuel economy. If you upgrade your wheels and tires, keep the old ones. I have fancy sport rims and aggressive tires on my own car, but I keep the stock wheels with a good narrower-tread performance tire in the garage. For long road trips, the stock wheels give a smoother ride and better economy.
8. Clean out your carIf you're the type who takes a leisurely attitude towards car cleanliness -- and I definitely fall into that category -- periodically go through your car and see what can be tossed out or brought into the house. It doesn't take much to acquire an extra 40 or 50 lbs. of stuff, and the more weight your car has to lug around, the more fuel it burns.
9. Downsize or hybridize
If you're shopping for a new car, it's time to re-evaluate how much car you really need. Smaller cars are inherently more fuel-efficient, and today's small cars are safer and roomier than ever. And if you've never considered a hybrid or a diesel, maybe it's time -- small hybrids like Toyota's compact Prius c (not to mention Honda's family-sized Accord Hybrid) are great in town, while diesels like the Volkswagen Jetta TDI and Chevrolet Cruze Diesel get great fuel economy on the open road.